Auto merge of #12740 - changm:readme, r=pcwalton

Update readme to tell users how to overcome compile error on OS X

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Just updates the README.md on how to get libavformat to link correctly on OS X for some systems.
---
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- [ X] `./mach build -d` does not report any errors
- [ X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ X] These changes do not require tests because it's a readme.

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README.md

@@ -34,6 +34,16 @@ brew install openssl
 brew link --force openssl
 ```
 
+If you get this error:
+``` sh
+"Couldn't find libavformat", do the following:
+
+brew uninstall ffmpeg
+brew install ffmpeg --build-from-source
+./mach clean
+./mach build
+```
+
 If you've already partially compiled servo but forgot to do this step, run ./mach clean, link openssl, and recompile.
 
 On Debian-based Linuxes: